Soft Tissue Fibroblastic Reticular Cell Tumor With Whole-Exome Sequencing Findings: An Unexpected Presentation of Lynch Syndrome

Fibroblastic reticular cell tumor (FRCT) is an exceedingly rare tumor that is histologically reminiscent of follicular dendritic cell sarcoma or interdigitating dendritic cell sarcoma but lacks the immunophenotypic features of these tumors. This tumor is classically described in lymph nodes and spleen, with only 4 cases described in soft tissues. We report a case of FRCT presenting as a right thigh mass in a 67-year-old woman with no prior malignancies. Gross examination showed a 10.6-cm tan well-circumscribed intramuscular mass. Microscopic examination revealed a discohesive population of cells with indistinct pale cytoplasm and large irregular, atypical vesicular nuclei with variably prominent nucleoli in a collagenous background infiltrated by lymphocytes. The tumor cells were positive for smooth muscle actin, cytokeratins (in a dendritic pattern), and CD163, while negative for CD21, CD35, and CD23, supporting the diagnosis of FRCT. Whole-exome sequencing revealed 631 putative somatic mutations in the tumor (>10 mutations/Mb of sequence). Mutational signature analysis suggested DNA mismatch repair deficiency. Germline mutational analysis revealed a heterozygous pathogenic missense mutation of MLH1 (c.2246 T > C, p.Leu749Pro). Subsequent immunohistochemical analysis showed complete loss of MLH1 and PMS2 in tumor cells. To our knowledge, this is the first case of FRCT characterized by sequencing studies and found to be associated with Lynch syndrome (LS), expanding the spectrum of LS-associated neoplasms. This case demonstrates genetic hypermutation similar to that seen in the more common epithelial lesions arising in LS, and it highlights the potential for high-throughput genetic analysis to identify mismatch repair-deficient tumors of atypical histologies, which may have significant clinical implications in the era of immunotherapy.